Police officers arrest a man on the eve of the 34th anniversary of China's Tiananmen Square massacre in Hong Kong. AP

Restrictions in Hong Kong have stifled what were once the biggest vigils marking the bloody crackdown by Chinese troops on pro-democracy demonstrators, leaving cities like London, New York, Berlin and Taipei to keep alive the memory on the June 4 anniversary.

Commemorations are expected on Sunday in at least 30 places in North America, Europe and Asia.

The eight people were detained near Victoria Park, where for years after 1989 democracy activists gathered on the Tiananmen Square anniversary.

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Among them was artist Sanmu Chan who chanted "Do not forget June 4. Hongkongers don’t be scared", as police led him away on Saturday evening, according to a video clip seen by Reuters.

The police said the eight had been detained after "displaying protest items loaded with seditious wordings, chanting and committing unlawful acts".

Hong Kong activists say such police action is part of a broad campaign by China to end dissent in the city that was promised special freedoms for 50 years under a "one country, two systems" formula when former colonial power Britain handed it back in 1997.

Police officers take away a member of the public in the Causeway Bay area in Hong Kong on Saturday. AP

Security is significantly tighter across Hong Kong this year, with up to 6,000 police officers deployed, including riot and anti-terrorism officers, the public broadcaster said.

Senior officials have warned people to abide by the law.

"Police are highly concerned about some people attempting to incite and provoke others to commit illegal acts that endanger national security, public order and public safety," police said in a statement.

In Beijing, Tiananmen Square was thronged with tourists taking pictures under the watchful eyes of police and other personnel but with no obvious sign of stepped-up security.

A group of relatives called the Tiananmen Mothers said the anguish never ended.

"Though 34 years have passed, for us, family members of those killed, the pain of losing our loved ones in that one night has tormented us to this day," the group said in a statement released by the New York-based watchdog Human Rights in China.

Reuters